Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a technique in which a transistor is manufactured using an amorphous oxide containing In, Zn, Ga, Sn, and the like as an oxide semiconductor is disclosed in Patent Document 1. Furthermore, a technique in which a transistor using an oxide thin film and a self-aligned top-gate structure is manufactured is disclosed in Patent Document 2.
Furthermore, a flexible device in which a functional element such as a semiconductor element, a display element, or a light-emitting element is provided over a substrate having flexibility (hereinafter also referred to as a flexible substrate) has been developed. Typical examples of the flexible device include, as well as a lighting device and an image display device, a variety of semiconductor circuits including a semiconductor element such as a transistor.
Patent Document 3 discloses a flexible active matrix light-emitting device in which an organic electroluminescence (EL) element and a transistor serving as a switching element are provided over a film substrate.
Display devices are expected to be applied to a variety of uses and become diversified. For example, a smartphone and a tablet terminal with a touch panel are being developed as portable information appliances.